1. Field of the Invention
The present invention relates to projection lens systems with a magnification-varying capability, and to projectors. More particularly, one or more embodiments of the present invention relates to zoom lens systems for projection that have a wide focus range suitable for projection of an image displayed on an image display device such as a digital micromirror device or an LCD (liquid crystal display) onto a screen on a magnified scale, and to projectors provided with such a zoom lens system.
2. Description of Related Art
Zoom lens systems of a type that is suitable as a projection lens system for projectors are proposed in Patent Documents 1 and 2 identified below. The zoom lens system disclosed in Patent Document 1 has a negative-led six-group design of a negative-negative-positive-negative-positive-positive type or a negative-negative-positive-negative-negative-positive type. The zoom lens system disclosed in Patent Document 2 has a negative-led five-group design of a negative-positive-negative-positive-positive type. The zoom lens system disclosed in Patent Document 1 adopts a focus system in which a first lens group is moved as a whole. On the other hand, the zoom lens system disclosed in Patent Document 2 adopts a so-called floating focus system, in which a plurality of lens groups are moved along different loci to achieve focusing. Specifically, the first lens group is divided into three sub-groups, and the foremost two groups are moved along different loci, thereby to achieve focusing while correcting variation of aberrations.    Patent Document 1: WO2014/104083 A1    Patent Document 2: JP-A-H08-005921
In the zoom lens system disclosed in Patent Document 1, variation of longitudinal chromatic aberration and curvature of field resulting from variation of the projection distance cannot be satisfactory corrected; this makes it difficult to give the zoom lens system a wide focus range. In the zoom lens system disclosed in Patent Document 2, as a result of a group with a large diameter being moved, focusing requires an increased torque; this may affect the durability of the focusing mechanism. In the focus systems proposed in Patent Documents 1 and 2, the lens group arranged at the most enlargement side is moved; this may inconveniently cause the lens total length to vary with focusing, leading to a large size in the mechanism that holds the focusing group.